The invention relates to an electric heating apparatus for heating foods or liquids. Normally, hot plates are used for this purpose and the cooking vessels are placed on the heating surface thereof.
The hot plates which are conventionally used in Europe and of which many millions have proved to be completely satisfactory are described for example in U.S. Pat. No. 4,122,330 and comprise a casting with coiled filaments housed in grooves in an embedding medium on the bottom thereof. While retaining the advantages of such hot plates, attempts are being made to improve them with a view to a lower thermal capacity and lower weight or material consumption. Although in the continuous state, the efficiency of the known hot plate is good, further energy could be saved by a lower thermal capacity in the case of short cooking processes (pre-cooking).
In addition, cooking elements are known comprising spiral tubular heating devices having a triangular cross-section and with an enlarged upper contact surface on which the cooking vessels are placed. Such hot plates are for example known in British Pat. No. 767,887 and U.S. Application Ser. No. 961,837, applied for on Nov. 17, 1978, inventor Gerhard Gossler. They have a very good efficiency, particularly for pre-cooking processes, due to their low thermal capacity. However, it is often considered disadvantageous that the heating devices are open and difficult to clean. Furthermore, in the construction according to British Pat. No. 767,887 the hot plate can be penetrated by overflowing food being cooked, which is very disadvantageous.
U.S. Pat. No. 2,299,596 describes a construction in which trapezoidal heating strips are connected via also trapezoidal intermediate members to a substantially planar plate. However, this plate is difficult to manufacture and has not proved successful under practical conditions.
Furthermore, U.S. Pat. Nos. 3,191,003, 3,632,983 and 3,789,189 disclose so-called glass ceramic cookers in which the cooking surface comprises a glass ceramic plate and which are heated from below by contact heating through triangular spirally wound tubular heating devices. Although these glass ceramic plates have the advantage of a closed upper cooking surface, which can optionally extend over several cooking positions, they have the disadvantage that the glass ceramic plate is a poor heat conductor and consequently the heat transfer from the bottom to the top and also the heat distribution are poor. The heating elements must be brought to very high temperatures in order to transfer sufficient energy and difficulties are encountered in the precise temperature regulation.
In order to bring about a better heat distribution, U.S. Pat. Nos. 3,674,983 and 3,686,477 disclose a glass ceramic heating system construction in which a sheet aluminium heat distribution plate is located between the glass ceramic plate and the heating elements and the tubular heating devices are soldered to the said aluminium plate. However, this construction must be very critical in operation due to the low melting temperature of aluminium and the high heating conductor temperatures necessary and does not improve the essential problems of the glass ceramic plate.
Finally, DOS No. 2,021,177 and U.S. Pat. No. 3,826,898 disclose attempts to create electric hot plates having an upper plate made from a composite material, for example copper between two surface layers of stainless steel and which are heated from below by tubular heating devices soldered thereto. Although the composite plate ensures a good heat distribution, it does not have a very low thermal capacity. Furthermore, difficulties are encountered in soldering the tubular heating devices on a mass production basis. This soldering is not carried out and thermal contact is poor.
Furthermore, German Utility Model 7,811,510 discloses a heating element in which the tubular heating device is firmly surrounded by a covering plate, but a great deal of heat is dissipated and lost.